Structure and dynamics of aqueous solutions containing poly-(acrylic acid) and non-ionic surfactant pentaethylene glycol n-octyl ether (C8E5): A molecular simulations study

Abstract

Self-assembly of non-ionic surfactant pentaethylene glycol n-octyl ether (C8E5) in aqueous solution containing anionic polyelectrolyte poly (acrylic acid) PAA was investigated via atomistic molecular dynamics (MD) simulations. The concentration of surfactant (Cs) varied in the range (Cs = 9.6 × 10−4–0.3 M) of dilute to concentrated systems while polymer concentration was in dilute solution condition (i.e. Cp = 0.01 M). Binary and ternary systems show formation of nearly spherical micellar aggregate at low Cs and lamellar aggregates at high Cs. Strong interaction between PAA and C8E5 is observed at low Cs, in agreement with experimental data. PAA chain adsorbs onto the micellar aggregate and wraps around the aggregate. For lamellar phase aggregates PAA chain shows polar interactions with hydrophilic moieties of surfactant, originating from interactions between COOH groups of PAA and CO groups of C8E5. The concentration at which free-micelles form in ternary (polymer, surfactant and water) solution is greater than CMC of the binary surfactant-water system, in qualitative agreement with experimental data. At low Cs the interaction between surfactants and polymer chain is weak and does not perturb chain conformations. The radius-of-gyration Rg of PAA increases with Cs. Interaction of surfactant molecules with water is more cooperative at higher values of surfactant concentration, as evident from the values of solvation enthalpy. Significant hydrogen bonds were observed to occur between polar oxygens and hydroxyl oxygen (O2HA) of PAA. The number of h-bonds between polar oxygens and O2HA of PAA increase with Cs. Hydrogen bonds of PAA with surfactant molecules increases with Cs as the polymer expands at high Cs. Formation of surfactant aggregate is enthalpically favored across the entire range of concentration in binary and ternary solution. Binding of PAA to surfactant aggregate is favorable at higher surfactant concentration. The enthalpy of formation of ternary solution exhibits a transition from unfavorable to favorable which interestingly is driven by the structural transition from spherical micellar to the lamellar phase. This signature for this transition is also seen by conformation change of PAA upon binding to surfactant aggregate.